Vital signs of a person, for example the heart rate (HR), the respiration rate (RR) or the blood oxygen saturation, serve as indicators of the current state of a person and as powerful predictors of serious medical events. For this reason, vital signs are extensively monitored in inpatient and outpatient care settings, at home or in further health, leisure and fitness settings.
One way of measuring vital signs is plethysmography. Plethysmography generally refers to the measurement of volume changes of an organ or a body part and in particular to the detection of volume changes due to a cardio-vascular pulse wave traveling through the body of a subject with every heart beat.
Photoplethysmography (PPG) is an optical measurement technique that evaluates a time-variant change of light reflectance or transmission of an area or volume of interest. PPG is based on the principle that blood absorbs light more than surrounding tissue, so variations in blood volume with every heart beat affect transmission or reflectance correspondingly. Besides information about the heart rate, a PPG waveform can comprise information attributable to further physiological phenomena such as the respiration. By evaluating the transmissivity and/or reflectivity at different wavelengths (typically red and infrared), the blood oxygen saturation can be determined.
Conventional pulse oximeters for measuring the heart rate and the (arterial) blood oxygen saturation of a subject are attached to the skin of the subject, for instance to a finger tip, earlobe or forehead. Therefore, they are referred to as ‘contact’ PPG devices. A typical pulse oximeter comprises a red LED and an infrared LED as light sources and one photodiode for detecting light that has been transmitted through patient tissue. Commercially available pulse oximeters quickly switch between measurements at a red and an infrared wavelength and thereby measure the transmissivity of the same area or volume of tissue at two different wavelengths. This is referred to as time-division-multiplexing. The transmissivity over time at each wavelength gives the PPG waveforms for red and infrared wavelengths. Although contact PPG is regarded as a basically non-invasive technique, contact PPG measurement is often experienced as being unpleasant, since the pulse oximeter is directly attached to the subject and any cables limit the freedom to move.
Recently, non-contact, remote PPG (RPPG) devices for unobtrusive measurements have been introduced. Remote PPG utilizes light sources or, in general radiation sources, disposed remotely from the subject of interest. Similarly, also a detector, e.g., a camera or a photo detector, can be disposed remotely from the subject of interest. Therefore, remote photoplethysmographic systems and devices are considered unobtrusive and well suited for medical as well as non-medical everyday applications.
Verkruysse et al., “Remote plethysmographic imaging using ambient light”, Optics Express, 16(26), 22 Dec. 2008, pp. 21434-21445 demonstrate that photoplethysmographic signals can be measured remotely using ambient light and a conventional consumer level video camera.
One of the main advantages of camera-based vital signs monitoring over on-body sensors is the high ease-of-use: there is no need to attach a sensor, just aiming the camera at the skin/chest of the subject is sufficient. Another advantage of camera-based vital signs monitoring over on-body sensors is the potential for achieving motion robustness: cameras have a significant spatial resolution while contact sensors mostly consist of a single element detector.
One of the key challenges for this technology is to be able to provide robust measurement under motion/light distortions. Several methods have been developed to enable robust camera-based vital signs measurement. For such measurements, usually a plurality of signals is captured based on image processing of captured image or image sequence. The plurality of signals may originate from different pixels of a sensor or also from different color channels of one pixel or of the same spatial position. Then, a photoplethysmographic waveform is formed based on a plurality of the signals. This waveform is the basis for further analysis, such as the determination of vital signs of a subject.
In WO 2013/027027 A2 there is disclosed a method for remotely monitoring vital signs by detecting a PPG signal in an image of a subject taken by a video camera such as a webcam. The PPG signal is identified by auto-regressive analysis of ambient light reflected from a region of interest on the subject's skin. Frequency components of the ambient light and aliasing artifacts resulting from the frame rate of the video camera are cancelled by auto-regressive analysis of ambient light reflected from a region of interest not on the subject's skin, e.g. in the background. This reveals the spectral content of the ambient light allowing identification of the subject's PPG signal.
In US 2013/041277 A1 a method for extracting the feature of an abdominal breathing and a system using the same are disclosed. The method is, capable of extracting the feature of an abdominal breathing, without the requirement of a standard model of an abdominal breathing and the execution of a learning process being executed prior to the method for extracting the feature of an abdominal breathing. By means of computing a plurality of intrinsic mode functions corresponding to the abdominal breathing signal received, an Euler angle function and an instantaneous frequency function of each of the plurality of intrinsic mode functions, and comparing the plurality of instantaneous frequency function with a pre-determined zero-point threshold region, the method for extracting the feature of an abdominal breathing defines one of the plurality of instantaneous frequency function as an abdominal breathing feature function, which contains the feature of the abdominal breathing. In this way, the feature of an abdominal breathing is extracted.
There exist various other approaches that rely on the spectral energy when it comes to the point of selecting the right set of signals to form a remote photoplethysmographic waveform i.e. a subject's PPG signal. This, however, implies that any distortion exhibiting strong frequency component inside the heart-rate bandwidth (0.5-3.5 Hz) may also be perceived as good candidate for the remote photoplethysmographic waveform generation. This often leads to unreliable waveforms.